1. Field of the Invention
The present invention relates to a charge/discharge circuit, and more specifically to a charge/discharge circuit which is used in combination with a secondary cell and which is incorporated in a power supply circuit associated to electronic instruments and appliances.
2. Description of Related Art
Referring to FIG. 1, there is shown a basic structure of a typical conventional power supply circuit associated to electronic instruments and appliances, which is shown as the prior art in FIG. 4 of Japanese Utility Model Application Laid-Open Publication No. Heisei 1-159550.
The shown power supply circuit is composed of a combination of a DC power supply (AC-DC converter) 11 receiving an AC 100V and generating a DC voltage output, and a secondary cell (rechargeable battery) 2, so that a load 6 is driven by either the DC voltage generated from the DC power supply 11 or a DC output of the secondary cell 2.
More specifically, a positive terminal of the DC power supply 11 is connected to an output terminal 12A, and a negative terminal of the DC power supply 11 is connected to another output terminal 12B. A positive electrode of the secondary cell 2 is connected to the output terminal 12A through a parallel circuit consisting of a resistor R and a diode D, and a negative electrode of the secondary cell 2 is connected to the output terminal 12B. An anode of the diode D is connected to the positive electrode of the secondary cell 2, and a cathode of the diode D is connected to the output terminal 12A. The pair of output terminals 12A and 12B are connected to the load 6.
Now, assuming that the DC power supply 11 is a so-called AC adaptor, if the AC adaptor is connected to the terminals 12A and 12B, the secondary cell 2 is charged through the resistor R by the AC adaptor, and simultaneously, an electric power is supplied to the load 6 from the AC adaptor. After the voltage of the secondary cell 2 becomes the same as the output voltage of the AC adaptor (DC power supply 11), the DC electric power generated in the AC adaptor is supplied only to the load 6. If the AC adaptor is disconnected, an electric power is supplied from the secondary cell 2 through the diode D to the load 6.
As will be apparent from the above, only when the output voltage of the DC power supply 11 is higher than the voltage of the secondary cell 2, it is possible to charge the secondary cell. This charging is controlled on the basis of only a resistance value of the resistor R and the output voltage of the DC power supply 11. Therefore, a protection circuit is required which is capable of preventing an overcharging of the secondary cell. On the other hand, if the AC adaptor (DC power supply 11) is disconnected, the electric power is supplied from the secondary cell 2 to the load 6. However, since a discharge path of the secondary cell 2 is composed of only the diode D, even if the secondary cell becomes in an overdischarged condition, the electric power may often continue to be supplied from the secondary cell. Accordingly, a circuit for preventing the overdischarging is also required. Particularly, in the case that a lithium secondary cell is used, since performance of the lithium secondary cell is extremely deteriorated by the overcharging and the overdischarging, the protection circuit is indispensable.